Waste water is generated from common sources, such as homes, schools, restaurants, hotels, office buildings, and the like. From these sources, waste water enters a collection system and is gravity fed downstream through underground sewer pipes to a municipal treatment plant where the waste water is chemically and biologically treated for return to the environment. Collection basins containing one or more waste water pumps are located in areas of low elevation between the sources of waste water and the treatment plant.
A pump station is generally constructed of concrete or fiberglass, and is typically between four to twelve feet in diameter, and can range from four to over forty feet deep. The pump station receives the flow of waste water from the gravity sewer pipes that feed it. The pump station also houses one or more discharge pumps that serve to “push” the waste water to another high point, or directly to the treatment plant. Depending on the plant location, multiple pump stations may be required to transport the waste water to its final destination for treatment.
Each pump station has some means of signaling when the discharge pumps should turn “on” and “off” depending on the level of waste water in the basin. Most commonly, this signaling means comprises four float switches staggered at different elevations in the basin. These floats are wired back to a central control panel that houses the motor starters for the pumps. Depending on which of the floats “tips” determines which of the pump motors energizes, when it “de-energizes”, and when there is a “high water” or “low water” condition.
Waste water entering these pump stations typically conveys many contaminants along with the water—in particular, fats, oils and grease (FOG). As waste water stored in the pump station becomes stagnant, most of this FOG rises to the surface and solidifies. This FOG tends to collect on, and build-up around, anything it touches. It is particularly common for grease to collect on the aforementioned floats that control the operation of the pumps. When grease collects on a float, it can weigh the float down or hold it in one position. By not allowing the float to “tip,” operation of the associated electrical switch is restricted, which in turn, affects automatic operation of the pump. Also, grease may “bridge” from a float to a pipe or other structure in the pump station, and the float may “stick” in the “up” position. When this happens, the pump motor never gets its signal to turn off, and continues to run even after there is no waste water remaining in the basin. In this event, the motor overheats causing substantial and costly mechanical damage to the pump.
Other methods of waste water level measurement include the measurement of electrical conductivity, as water is a conductive liquid. One method of conductivity utilizes a rod having a plurality of metal contacts on the rod, the rod being located in the pump station. Each rod contact is connected to a unit in the control panel which applies a low AC voltage to each contact and checks each for a current to ground above a certain user-defined threshold. Thus, the level in the well can be determined depending on the number of contacts allowing current to flow to ground.
As can be appreciated, when FOG and other waste material builds up on the rod the waste materials can form an imperfect electrical seal that bridges one or more of the metal contacts. The waste material bridge serves to retain liquid, and thus enables an electrical connection between the metal contact and ground. This bridge-like structure of the waste material can continue to hold liquid in suspension. Or, after the water level has dropped and the bridge has somewhat dried out, capillary action may bring liquid from rising waste water up through the waste material. In either of these cases, once the liquid in the well has risen up to the bottom of the waste material bridge, an electrical path capable of conducting the electrical measurement current will be created between ground and all of the metal contacts the waste material has formed over. So the electronic unit may detect a false level—higher than the actual waste water level.
Therefore, when a lower contact is covered with liquid, it also provides an electrical path to a higher contact. This condition produces a false reading and “short cycling” of the pumps in that a waste water pump will begin operation sooner than it normally would. The result is that the waste water pumps cycle on and off more frequently than needed, leading to both short-term and long-term pump problems.
A similar problem occurs with the conductivity device as FOG builds up on the rod, especially around the contact which corresponds to the pump starting. This is because it is the highest level that is usually reached in the pump station. As the FOG is non-conductive it can prevent an electrical current from flowing even when the liquid level reaches the same level as this contact. In this case, the level will keep rising until a higher contact is reached, but the same problem will eventually occur on this higher contact. In this scenario, the pump motor may not activate to transfer the rising waste water from the pump station further downstream towards the treatment plant. As a result, the pump station may go into an “alarm” state, and can overflow onto the ground around the well, and into a nearby creek or stream.
All of these problems noted with conductivity devices require that the apparatus be cleaned with some frequency. Unfortunately there is no clear indication with existing devices whether or not cleaning is required, and as a large buildup sometimes has no detrimental effect the operators often clean the rod when it is not required.
Another shortfall of the conductivity device, and of floats, is that the level value is measured in steps. With ten contacts on a conductivity device, for example, there are at most only ten level “steps” that can be measured. Usually the rod is placed in the bottom half or bottom third of the well as this is where the pumps are ideally started and stopped. But once the water level progresses above the highest contact, or highest contact, on the rod, there is no way of knowing exactly what the level is. This is a problem in high level conditions as multiple wells may be in a high level condition. The operators need to know which station is most likely to overflow. And in variable speed drive (known as VSD or VFD) stations, which are becoming more common, even in the active range of the well the system needs an analog level reading, and not a stepped (discontinuous) level reading. VFDs do not work well with discontinuous “process variables.”